A press assembly for bending rectangular metal plates into U-profiles in the production of pipe sections conventionally includes a supporting frame, a vertically reciprocatable press beam, a pair of further press members operative in opposite directions in a horizontal plane, a centering device and a hydraulic drive with at least two actuating cylinders for each horizontal press member and with a synchronizing control. The plates to be bent frequently have a thick cross section and are highly resistant to bending stresses, and yet the U-profiles must be formed with very limited tolerances.
Generally, the synchronizing control comprises mechanical elements such as torsion shafts and racks and pinions which unavoidably have a play leading to undesirably large errors in the formation of the pipe sections. Furthermore, the centering device works independently of the hydraulic cylinders of the horizontal-press drive and consists of a combination of positioning spindles and hydraulic cylinders having their own characteristic error margins which become superimposed on the error due to the synchronizing control. Pipe sections with unacceptable deviations in dimensions must be either scrapped or specially handled. It is also notable that in known bending assemblies the horizontally operating press members are powered by a common pump.